A User Equipment (UE) transmitting a D2D signal can only operate in a half-duplex mode, but can not detect a D2D signal of another UE in a sub-frame in which the UE transmits the D2D signal, so if the D2D signals of the two UEs are transmitted in the same sub-frame, then they will not discover each other in the sub-frame. Furthermore if they are still transmitted over the same frequency resource, then they will interfere with each other due to resource collision. If they are transmitted over different frequency resources, then the probability that the other UEs detect their D2D signals successfully will drop due to in-band emission interference. The overall performance of D2D transmission in the system will drop in either of the scenarios.
The prior art will be introduced below in details.
Device to Device (D2D) communication refers to direct communication between UEs, where the adjacent UEs can transmit data over a direct link in a short-distance range so that the data will not be forwarded by a central node (i.e., a base station), as illustrated in FIG. 1. The D2D technology has the following advantages due to its characteristic of short-distance communication, and direct communication mode:
The UEs can communicate directly over a short distance to thereby achieve a higher data rate, a shorter delay, and lower power consumption;
Spectrum resources can be utilized efficiently by the UEs widely distributed in the network due to the short distance characteristic of the D2D communication link;
The direct communication mode of D2D can be adapted to a demand of, e.g., wireless Peer to Peer (P2P) and other services, for sharing local data so as to provide a flexibly adaptive data service; and
The large number of UEs widely distributed in the network can be utilized in direct D2D communication to extend a coverage area of the network.
The LTE D2D technology refers to D2D discovery and communication procedures, controlled by an LTE network, operating in an LTE licensed frequency band. On one hand, the D2D technology can make full use of the original advantages of the D2D technology, and also some problems of the traditional D2D technology, e.g., uncontrollable interference, etc., can be overcome due to the controlling by LTE network. The introduction of the LTE D2D characteristic enables the LTE technology to evolve from the pure wireless mobile cellular communication technology toward the Universal Connectivity Technology (UCT).
The D2D technology includes a D2D discovery and D2D communication, and allocation of D2D transmission physical resources will be described below taking a D2D discovery as an example.
During a D2D discovery, the UE needs to know both a receive resource area (in which the UE receives a discovery signal of another UE) and a transmit resource area (in which the UE transmits its own discovery signal), but it is impossible for the UE to both transmit and receive the discover signals in the same sub-frame due to its hardware restriction. Typically, system discovery resources include a set of sub-frames or a set of Physical Resource Blocks (PRBs), and a periodicity at which the set of sub-frames or the set of PRBs occurs, where the periodicity is a system discovery resource periodicity, as illustrated in FIG. 2. A system discovery resource periodicity can include several sub-frames, each of which includes several PRBs, and typically, these sub-frames or PRBs are consecutive uplink or downlink resources (e.g., consecutive uplink sub-frames). In each discovery resource periodicity, the UE can detect, in each sub-frame in which the discovery signal of the UE is not transmitted, the discovery signal of the other UE. If the UE is in the coverage area of the network, then the system discovery resources are typically configured by the base station, and if there is no network coverage, the system discovery resources can be configured by a cluster head or can be predefined. Those system discovery resources (i.e., sub-frames) over which the UE is allowed to transmit its own discovery signal can also be configured by the network or the cluster head, or determined under a prescribed rule. Particularly one of the system discovery resources over which the UE transmits the discovery signal can be selected by the UE from the allowable resources (i.e., a type 1 of discovery), or can be configured by the base station (i.e., a type 2 of discovery).
During the D2D discovery, the UE can only operate in the half-duplex mode, but can not detect the discovery signal of the other UE in the sub-frame in which the UE transmits the D2D signal, so if the D2D signals of two UEs are transmitted in the same sub-frame, then they will not discover each other in the periodicity. Furthermore if they are still transmitted over the same frequency resource, then they will interfere with each other due to resource collision; and if they are transmitted over different frequency resources, then the probability that the other UEs detect their D2D signals successfully will drop due to in-band emission interference. Typically if there is a smaller spacing between the frequency resources of the UEs transmitting the signals in the same sub-frame, then there is a more serious in-band emission interference.
In the discussion of D2D in the prior art, there has been proposed a resource hopping pattern between the discovery resources of the UEs in different discovery resource periodicities so that there is a fixed resource hopping relationship between the discovery resources of the UE in the different discovery resource periodicities, and thus the D2D UEs transmitting over different frequency resources in the same sub-frame can subsequently discover each other while transmitting over discovery resources in different sub-frames. For example, the following resource hopping pattern is applied to the UE:SF(i)=[floor(PRB(0)/nSF)*i*i+mod(PRB(0),nSF)*i+SF(0)] mod nSFPRB(i)=[PRB(0)+3*i] mod nRB
Where SF(i) represents the index of a sub-frame where a discovery resource of the UE is located in a discovery resource periodicity with the index i; PRB(i) represents the index of a PRB where the discovery resource of the UE is located in the discovery resource periodicity with the index i, nSF represents the total number of discovery sub-frames in a discovery resource periodicity, and nRB represents the total number of PRB resources in a discovery resource periodicity.
However in the existing resource hopping solution, the UEs occupying the different frequency resources in the same sub-frame can discover each other in the subsequent transmission, but the different UEs occupying the same frequency resource in the same sub-frame may still collide with each other in the subsequent transmission and thus can not discover each other, thus degrading the overall discovery probability of the system. Moreover in the existing hopping solution, the receiving end can not know from a physical resource over which the D2D signal is detected at some instance of time, a physical resource over which the UE sends the D2D signal next time, but has to perform blind detection again next time, thus increasing the complexity at which the UE subsequently detects the D2D signal.